One size doesn't fit all when it comes to breast shields, the plastic suction cups that fit over your nipples while pumping breast milk. Shields that fit correctly make pumping more comfortable and efficient. If you've been using the standard-size shields that came with your Medela pump, and your nipples aren't enclosed by the shields, or they get irritated while pumping, you may need bigger breast shields. Medela makes two sizes: large and extra-large. To determine which is best for you, hold a quarter over your erect nipple (the way it looks when your baby is nursing). If your erect nipple is the same size as the quarter, order the large shields. If your nipples are larger than the quarter, you'll need Medela's extra-large breast shield.